This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The major goals of this project have been to (1) determine whether sequence variants within gp120 are selected during transmission;(2) determine whether the selection of viral variants reflects the population of virus present in the genital fluids of the donor or a biological restriction of the transmission process;and (3) determine the frequency, kinetics, and the virologic and immunologic ramifications of HIV superinfection in both partners following early infection. The results of these studies are enhancing our understanding of the heterosexual transmission process and yield novel information that is critical to the design and testing of globally effective vaccine candidates. We are analyzing HIV-1 virus populations, isolated from both the donor and recipient immediately following a transmission event, in unique cohorts of discordant couples in Rwanda and Zambia. These studies have provided a unique opportunity to investigate the virologic determinants of heterosexual transmission using samples from large, well-characterized discordant couple cohorts that represent two predominant clades (A and C) of HIV-1. Recent studies show clade C but not clade A viruses, which establish infection, encode a more compact Env protein compared to donor derived quasispecies. The viral transmission is primarily monophyletic, but inflammatory genital infections are often associated with the transmission of multiple quasispecies, suggesting an intact mucosa represents a strong barrier to infection. Env isolated from both partners showed strong requirements for both CD4 and CCR5 receptors, but enhanced infection of cells with low receptor/coreceptor levels does not occur with more compact Envs as hypothesized. Phylogenetic analysis shows transmitted viruses do not correspond to more abundant quasispecies in donor genital tracks, but instead represent minor species. Finally, superinfection was detected in 3 of 15 unlinked (transmission outside marriage) couples suggesting an opportunity for more rapid viral evolution through recombination.